Write the authors of the simulation paper that has a "correct" APL for POPE and 
ask them for an input file.
That is really the only way to be sure that you are not doing something 
different than they did.
In my experience, people are quite willing to provide you with their input 
file(s).
If you still get a different APL than they reported, then see if your 
simulation times are similar and repeat your run
a few times to see if it's just statistical noise.

Regarding 323 K, I don't recall... it's just a number that sticks in my head. 
Perhaps it is for DPPE or DPPC.

I'd still suggest that you at least try POPC. So your peptide binds more 
favourably to POPE than to POPC... 
that alone does not limit you to POPE. Then again, I don;t know exactly what 
you are trying to do.

Chris.


-- original message --


My peptide is known to be more favorably to PE than PC membrane that is why I 
am using POPE.

Experimentally, the liquid phase transition is at 298K for POPE (if I am not 
mistaken). Is your 323K refer to some simulations? 

At first I wanted to use the new CHARMM36 lipids parameters because they are 
supposed to solve the previous CHARMM27 issue with the area per lipid. However, 
I am consistently obtained smaller APL then experiment and I am not able to 
reproduce the published APL obtained for POPE, even if I am starting from their 
equilibrated 80-POPE membrane and use same simulation conditions. That was the 
reason for starting this thread on the mailing list. 

Unfortunately, my peptide conformational space in solution is only 
well-represented by CHARMM27 (equivalently in CHARMM36), so I can not use 
Berger's lipid parameters with OPLS or GROMOS even if it would be preferable as 
they do not have APL inconsistency and are united-atom.

I will made some tests in the NPAT ensemble. Perhaps the NPAT effects can be 
made neglegible by using bigger membrane compared to my peptide's size (?). 

Sebastien 
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